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Overview

Problemize implements centralized exception handling through the ExceptionHandler class, which integrates seamlessly with ASP.NET Core’s global exception handling pipeline. This approach ensures that all unhandled exceptions in your Web API are converted into standardized Problem Details responses.

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How it works

The ExceptionHandler class implements ASP.NET Core’s IExceptionHandler interface, making it part of the framework’s built-in exception handling middleware.

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Core architecture

The handler depends on two key components:
Services/ExceptionHandler.cs
public sealed class ExceptionHandler : IExceptionHandler
{
    private readonly IProblemDetailsService _problemDetailsService;
    private readonly IStatusCodeMapper _statusCodeMapper;

    public ExceptionHandler(
        IProblemDetailsService problemDetailsService, 
        IStatusCodeMapper statusCodeMapper)
    {
        _problemDetailsService = problemDetailsService;
        _statusCodeMapper = statusCodeMapper;
    }
}
IProblemDetailsService is ASP.NET Core’s built-in service for writing Problem Details responses, while IStatusCodeMapper is Problemize’s interface for mapping exceptions to HTTP status codes.

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Exception processing flow

When an unhandled exception occurs, the TryHandleAsync method executes the following steps:

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1. Map exception to status code

Services/ExceptionHandler.cs
httpContext.Response.StatusCode = _statusCodeMapper.GetStatusCode(exception);
The status code mapper determines the appropriate HTTP status code based on the exception type.

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2. Create problem details context

Services/ExceptionHandler.cs
var problemDetailsContext = new ProblemDetailsContext
{
    HttpContext = httpContext,
    Exception = exception,
};
This context object carries all the information needed to generate the response.

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3. Build the problem details object

The handler creates different problem details objects based on the exception type:
Services/ExceptionHandler.cs
if(exception is ValidationException validationException)
{
    problemDetailsContext.ProblemDetails = new ValidationProblemDetails
    {
        Title = "An error occured while validating your request",
        Detail = validationException.Message,
        Type = validationException.GetType().Name,
    };
}
else
{
    problemDetailsContext.ProblemDetails = new ProblemDetails
    {
        Title = "An error occured while processing your request",
        Detail = exception.Message,
        Type = exception.GetType().Name,
    };
}
ValidationException instances receive special handling with ValidationProblemDetails, which supports structured validation error responses.

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4. Write the response

Services/ExceptionHandler.cs
return await _problemDetailsService.TryWriteAsync(problemDetailsContext);
The IProblemDetailsService serializes the problem details object and writes it to the HTTP response, applying any customizations configured in your application.

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Registration

The exception handler is automatically registered when you call UseExceptionHandling:
Configurator.cs
services.AddExceptionHandler<ExceptionHandler>();
This registers the handler with ASP.NET Core’s dependency injection container and integrates it into the exception handling pipeline.
The exception handler runs before other exception handlers in the pipeline. If it returns true from TryHandleAsync, the exception is considered handled and won’t propagate to other handlers.

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Key benefits

  • Automatic conversion: All exceptions are automatically converted to Problem Details format
  • Type-aware handling: Different exception types receive appropriate status codes and response formats
  • Pipeline integration: Works seamlessly with ASP.NET Core’s exception handling middleware
  • Extensible: The status code mapper can be customized for application-specific exception types

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